Counterbalancing system for a drafting table or the like

ABSTRACT

In a drafting table having a fixed support, an arm pivotally connected to the support about a first horizontal axis, and a drawing board pivotally connected to the arm about a second horizontal axis which is spaced from but parallel to the first axis, there is provided a counterbalancing system comprising cam members and spring members operably interconnected with the support, the arm, and the drawing board, the counterbalancing system, within a predetermined range of angular settings of the drawing board about the second horizontal axis and of the arm about the first horizontal axis, substantially counterbalancing the turning moments exerted by the weight of the arm and the drawing board about their respective axes as well as compensating for turning moments exerted about said axes by portions of the counterbalancing system itself.

[451 Sept. 30, 1975 United States Patent [191 Horner @H 8 Q0 1. 4 A b COUNTERBALANCING SYSTEM FOR A 3.239l84 3/1966 Kirkeby DRAWING TABLE OR THE LIKE 3,267,878 8/1966 Faux et al 3,359,927 12/1967 Janus [75] Inventor: George Irwin Horner, Toronto,

Canada Primary E.\'aminerRoy D. Frazier 73 Assignee; Onway Construction Company, Assistant Examiner-William E. Lyddane Limited, Etobicoke, Canada Sept. 21, 1973 [57] ABSTRACT In a drafting table having a fixed support, an arm piv- [22] Filed:

App! 399514 otally connected to the support about a first horizontal axis, and a drawing board pivotally connected to the arm about a second horizontal axis which is spaced from but parallel to the first axis, there is provided a counterbalancing system comprising cam members In .mm r PM K 0 PU D. n n9 g] .l r5 0; t 0. e S

52 us. c1. l08/2' 108/4- 108/6- and Spring members operably imerwnnected with the support, the arm, and the drawing board, the counterbalancing system, Within a predetermined range of angular settings of the drawing board about the second horizontal axis and of the arm about the first horizontal axis, substantially counterbalancing the turning moments exerted by the weight of the arm and the drawing board about their respective axes as well as 61 9 U6 ;O W4 w 1 a 000 2 O 00 2 m2 6 m3 m7. H n R2 an 5 MN 00 MO .w F l 00 5 References Cited UNITED STATES PATENTS compensating for turning moments exerted about said.

axes by portions of the counterbalancing system itself.

2.584.921 Rawnsley et 2.988343 Knudsen l4 C a ms, 9 Drawing Figures U.S. Patent Sept. 30,1975 Sheet 1 0f 3 US. Patent Sept. 30,1975 Sheet 2 of3 3,908,560

FIG. 2

US, Patent Sept. 30,1975 Sheet 3 0f 3 8560 ll I.

COUNTERBALANCING SYSTEM FOR A DRAFIING TABLE OR THE LIKE This invention is concerned with apparatus incorporating pivotally connected members comprising a first member, a second member pivotally connected to the first member about a first horizontal axis, and a third member pivotally connected to the second member about a second horizontal axis which is spaced from but parallel to the first axis, in which the second and third members may, by pivotal movement thereof about the first horizontal axis relative to the first member, occupy positions in which the combined weight thereof exerts a turning moment about the first horizontal axis, and in which the third member may, by pivotal movement thereof about the second horizontal axis. relative to the second member, occupy positions in which the weight of the third member exerts a turning moment about the second horizontal axis. Apparatus of this type may be constituted by, for example, a drafting table with the first member comprising a fixed support, the second member comprising an arm, and the third member comprising a drawing board, although it is to be emphasized that the present invention is not restricted in scope to the apparatus necessarily being constituted by a drafting table.

It is conventional to provide in apparatus of the type hereinbefore referred to a first counterbalancing system which is intended to counterbalance the turning moment exerted by the combined weight of the second and third members about the first horizontal axis, and a second counterbalancing system which is intended to counterbalance the turning moment exerted by the weight of the third member about the second horizontal axis, apparatus of the type hereinbefore referred to and incorporating such counterbalancing systems being disclosed in, for example, US. Pat. No. 3,359,927 which issued on Dec. 26, 1967 to Stanley Janus.

There is, however, a tendency in such apparatus for hunting to occur if one of the members which are pivotally connected about the horizontal axes is pivotally moved about the appropriate axis. Thus, if, for example, the pivotal connection about the first horizontal axis between the first and second members is locked and the pivotal setting of the third member is varied by pivotal movement thereof relative to the second member about the second horizontal axis there is a tendency, if the pivotal connection about the first horizontal axis between the first and second member is subsequently unlocked, for the second member pivotally to move relative to the first member about the first horizontal axis under the influence of the variation in the turning moment about the first horizontal axis resulting from the variation in the pivotal setting of the third member. Any such pivotal movement of the second member relative to the first member about the first horizontal axis results in a further pivotal movement of the third member relative to the second member about the second horizontal axis, and so on.

It is a primary object of the present invention substantially to obviate or mitigate this disadvantage involving the tendency for hunting by substantially ensuring that in apparatus of the type hereinbefore referred to having a first counterbalancing system operable relative to the first horizontal axis and a second counterbalancing system operable relative to the second horizontal axis, variations in the turning moment about the first horizontal axis resulting from variations in the pivotal setting of the third system are substantially compensated by variations in the first counterbalancing system.

Apparatus according to the present invention comprises a first member, a second member pivotally connected to the first member about a first horizontal axis, a third member pivotally connected to the second member about a second horizontal axis which is spaced from about parallel to the first axis, a first counterbalancing system operable relative to the first horizontal axis, and a second counterbalancing system operable relative to the second horizontal axis, each counterbalancing system comprising a counterbalancing cam member having a cam surface, and a spring member which is disposed around the cam surface of the counterbalancing cam member and which acts between the two members which are pivotally connected about the respective horizontal axis, with the counterbalancing cam member being fixed relative to one of said two members, wherein a further cam member having a cam surface is fixed relative to the third member with the spring member of the first counterbalancing system disposed around the cam surface of said further cam member, and wherein, within a predetermined range of angular settings of the third member about the second horizontal axis and of the second member about the first horizontal axis, the algebraic sum of:

(W X d,,) (E, X d (F,, X d is substantially zero, and the algebraic sum of:

(W X d (W d,,) (F X d;) is substantially zero in which W weight of the third member,

W weight of the second member,

F force exerted by the spring member of the second counterbalancing system on the counterbalancing cam member thereof,

F force exerted by the spring member of the first counterbalancing system on said further cam member,

F force exerted by the spring member of the first counterbalancing system on the second member,

d perpendicular'distance of the second horizontal axis from the line of action W through the centre of gravity of the third member,

d perpendicular distance of the second horizontal axis from the line of action of P a perpendicular distance of the second horizontal axis from the line of action of F,,,

d perpendicular distance of the first horizontal axis from the line of action of W through the centre of gravity of the third member,

d perpendicular distance of the first horizontal axis from the line of action of W through the centre of gravity of the second member, and

d perpendicular distance of the first horizontal axis from the line of action of P In order that the invention may be more clearly understood and more readily carried into effect the same will now, by way of example, be more fully described with reference to the accompanying drawings in which FIG. 1 is an isometric view partially in phantom of apparatus constituted by a drafting table according to a preferred embodiment of the present invention;

FIG. 2 is a side view on an enlarged scale in the direction of the arrow A in FIG. 1 of parts of the drafting table shown therein, certain of these parts being shown for clarity in chain-dotted lines;

FIG. 2A is a sectioned view of an enlarged scale on the line 2A 2A in FIG. 2;

FIG. 3 is a view corresponding to FIG. 2 but with the parts of the drafting table illustrated therein in different relative positions;

FIG. 4 is a further view corresponding to FIG. 2 but with the parts of the drafting table illustrated therein in still different relative positions;

FIG. 5 is a still further view corresponding to FIG. 2 but with the parts of the drafting table illustrated therein in yet still different relative positions;

FIG. 6 is a view on an enlarged scale in the direction of the arrow B in FIG. 1 of further parts of the drafting table shown therein;

FIG. 7 is a side view, on an enlarged scale, in the di rection of the Arrow C in FIG. 1 of still further parts of the drafting table shown therein; and

FIG. 8 is a sectioned view on the line 88 in FIG. 7.

Referring to the drawings, 10 denotes generally apparatus which is constituted by a drafting table and which comprises a first member 11 in the form of a fixed support, a second member 12 in the form of an arm, and a third member 13 in the form of a drawing board. The fixed support 11 comprises a horizontal base plate 14 from the opposed ends of which upright members 15 project, the base plate 14, or spaced feet (not shown) presented by the lower face thereof, serving operatively to support the drafting table 10 on a horizontal support floor or the like. If such feet (not shown) are presented by the base plate 14 these feet may be adjustably mounted to ensure that the feet are operatively in contact with the support floor or the like irrespective of any deviations therein from the horizontal, and to ensure that the axes about which the arm 12 is pivotally connected to the support 11 and about which the board 13 is pivotally connected to the arm 12, as is hereinafter more fully described, are horizontal.

As is indicated above, the arm 12, which is in the form of a hollow casing, is pivotally connected to the support 11 about a first horizontal axis which is denoted by the reference numeral 16, a horizontal tube 17 being disposed between and fixed to the upright members 15 with the arm 12journalled on this tube 17 for pivotal movement of the arm 12 relative to the support 11 about the axis 16 which is constituted by the longitudinal axis of the tube 17. As is also indicated above, the board 13 is likewise pivotally connected to the arm 12 about a second horizontal axis which is de noted by the reference numeral 18, the main underface 19 of the board 13 presenting brackets 20 between which there is disposed a horizontal tube 21 which is fixed relative to the brackets 20 and on which the arm 12 is journalled, with the longitudinal axis of the tube 21 constituting the second horizontal axis 18. The second horizontal axis 18 is spaced from but parallel to the first horizontal axis 16.

There is provided a first counterbalancing system which is operable relative to the first axis 16, and a second counterbalancing system which is operable relative to the second axis 18.

The first counterbalancing system comprises a counterbalancing cam member 22 which is disposed within the arm 12 and which is fixedly mounted on the tube 17 so that this carn member 22 remains fixed at all times relative to the support 11. The first counterbalancing system also comprises a spring member comprising a flexible member 23, a coil spring 24, a further flexible member 25 and a lever 26, the flexible members 23 and 25 being constituted by flexible straps or cables. One end of the member 23 is secured at the location 27 to the cam surface 28 of the cam member 22 and the other end of the flexible member 23 is connected to one end of the spring 24, the other end of which is connected to one end of the further flexible member 25 with the other end of this member 25 being secured at the location 29 to one end of the lever 26. The other end of the lever 26 is pivotally mounted to the arm 12 about a pivot pin 30 the axis of which is spaced from but parallel to the first and second axes 16 and 18, respectively. Thus, the spring member constituted by the flexible members 23 and 25, the spring 24 and the lever 26 can be considered as being disposed around the cam surface 28 of the cam member 22 and as acting between the support 11 and the' arm 12.

Fixedly mounted on the tube 21 is a further or compensating cam member 31 the purpose and function of which is hereinafter more fully explained. This further cam member 31 is, in the preferred embodiment of the invention illustrated in the accompanying drawings, in the form of two spaced, parallel cam member portions 32 between which the lever 26 is pivotally movable, the lever 26 presenting between the ends thereof two freely rotatable bearing rollers 33 which are in bearing contact with the portions of the cam surface 34 of the cam member 31 which are presented by the two cam member portions 32. Thus, the lever 26 can be considered to be, between the ends thereof, in bearing contact with the cam surface 34 of the further cam member 31 so that the spring member constituted by the flexible members 23 and 25, the spring 24 and the lever 26 can be considered to be disposed around the cam surface 34 of the further cam member 31.

The second counterbalancing system comprises a counterbalancing cam member 35 which, in the preferred embodiment of the present invention illustrated in the accompanying drawings, is constituted by a cam member presenting two cam lobes 36 and 37, the cam member 35 which is disposed within the arm 12 being fixedly mounted on the shaft 21 so that this cam member 35 remains fixed at all times relative to the board 13. The cam member 35 has a cam surface 38 portions of which are, of course, presented by the cam lobes 36 and 37. The second counterbalancing system also comprises a spring member comprising a flexible member 39 which is in the form of a flexible strap or cable, and a coil spring 40, one end of vthe flexible member 39 being secured to the cam member 35 between the cam lobes 36 and 37 thereof and the other end of the flexible member 39 being connected to one end of the spring 40 the other end of which is connected to the arm 12 by a pin 41. Thus, the spring member constituted by the flexible member 39 and the spring 40 can be considered as being disposed around the same surface 38 of the cam member 35 and as acting between the arm 12 and the board 13.

With particular reference to FIG. 2 of the drawings, it will be noted that in this view the arm 12 is in the pivotal setting thereof about the first axis 16 in which the second axis 18 is in the same horizontal plane as the first axis 16. Furthennore, as shown in this view the board 13 is in the pivotal setting thereof about the second axis 18 in which the centre of gravity of the board 13 is disposed directly vertically above the second axis 18. Since in the preferred embodiment of the invention shown in the accompanying drawings the board 13 extends a greater distance to one side of the second axis 18 than to the other side thereof, i.e. an imaginary plane which contains the second axis 18 and which is at right angles to the plane of the board 13 intersects the board 13 at a position spaced from the centre thereof, as is most clearly shown in FIG. 1, and since the second axis 18 is offset from the plane of the board 13 it will be noted that with the centre of gravity of the board 13 directly vertically above the second axis 18 the plane of the board 13 is somewhat inclined as shown in FIG. 2.

With the board 13 in this pivotal setting there is of course no tendency for the board 13 to pivot in one direction or the other about the second axis 18, and as shown in FIG. 2 any tension exerted by the spring 40 which with the board 13 in this pivotal setting may be in a substantiallly relaxed condition acts substantially through the second axis 18 so that there is substantially no turning moment exerted by the second counterbalancing system on the board 13 about the second axis 18. Furthermore, in the condition shown in FIG. 2 the roller 33 acts on the cam surface 34 of the further cam member 31 at a portion thereof which is substantially arcuate relative to the second axis 18 so that any force exerted by the roller 33 on the further cam member 31 as a result of the influence of the spring 24 likewise produces substantially no turning moment on the further cam member 31, and hence on the board 13, about the second axis 18.

Considering the turning moments about the first axis 16 relative to the support 11, the weight of the board 13 acts, of course, in a vertically downward direction through the second axis 18 and so the combined weight of the board 13 and of the arm 12 through its centre of gravity tend pivotally to move the arm 12, together with the board 13, in a counterclockwise direction relative to the support 11 about the first axis 16 as viewed in FIG. 2, the flexible member 23 being, however, wrapped around the cam surface 28 of the cam member 22 so that the spring 24 exerts on the arm 12 a turn ing moment relative to the support 11 about the first axis 16 which is in a clockwise direction as viewed in FIG. 2 and which substantially counterbalances the turning moment exerted by the combined weight of the arm 12 and the board 13 in the counterclockwise direction about the first axis 16.

Referring now to FIG. 3 of the accompanying drawings in which the arm 12 is in the same pivotal setting relative to the support 11 about the fist axis 16 as is hereinbefore described with reference to FIG. 2, it will be noted that the board 13 has been pivotally moved relative to the arm 12 about the second axis 18 to a pivotal setting in which the centre of gravity thereof is disposed to the right of the vertical plane containing the second axis 18. Thus, with the board 13 in this pivotal setting shown in FIG. 3 there is, of course, a tendency for the board 13 under the influence of the weight thereof to pivot about the second axis 18 in a clockwise direction as viewed in FIG. 3. Moreover as a result of the influence of the spring 24, the roller 33 exerts on the cam surface 34 of the further cam member 31 a force which results in a turning moment in a clockwise direction about the second axis 18 being applied to the further cam member 31, and hence to the board 13.

However, with the board 13 in this pivotal setting shown in FIG. 3 the flexible member 39 is wrapped around the cam surface 38 of the cam member 35, and more particularly around the cam lobe 36 thereof, so that the spring 40 exerts on the board 13 a turning moment about the second axis 18 which substantially counterbalances the turning moment exerted on the board 13 by the weight thereof, and by the force exerted by the roller 33 on the cam surface 34 of the further carn member 31, in the clockwise direction about the second axis 18.

Considering the turning moments exerted about the first axis 16 relative to the support 11, these are, of course, identical to the turning moments as hereinbefore described with particular reference to FIG. 2 since the arm 12 is still in the pivotal setting in which the sec ond axis 18 is in the horizontal plane containing the first axis 16, except that the variation as hereinbefore referred to in the position of the centre of gravity of the board 13 results in a variation in the turning moment about the first axis 16. As shown in FIG. 3 the above variation in the position of the centre of gravity of the board 13 has resulted in a reduction in the counterclockwise turning moment being exerted by the weight of the board 13 about the first axis 16 relative to the support 11, but underthe influence of the cam surface 34 of the further cam member 31 on the bearing rollers 33 the lever 26 is so pivotally moved about the pivot pin 30 as to relax the spring 24 by an amount such that there is a reduction in the clockwise turning moment exerted on the arm 12 by the spring 24 about the first axis 16 relative to the support 11 which is substantially equal to the above-mentioned reduction in the counterclockwise turning moment exerted by the weight of the board 13 about the first axis 16 relative to the support 11. Thus, the further cam member 31 serves as a compensating cam member to ensure that variations in the turning moment about the first axis 16 resulting from variations in the pivotal setting of the board 13 are substantially compensated by variations in the first counterbalancing system.

Referring now to FIG. 4 of the accompanying drawings, it will be noted that in this view the arm 12 has been pivotally moved about the first axis 16 relative to the support 11 to the pivotal setting thereof about the first axis 16 in which the second axis 18 is in the same vertical plane as the first axis 16, the board 13 being in the same pivotal setting about the second axis 18 as is hereinbefore described with reference to FIG. 2. Thus, as in the case hereinbefore described with reference to FIG. 2 there is no tendency for the board 13 to pivot in one direction or the other about the second axis 18, but since, because of the pivotal setting of the arm 12 relative to the support 11 about the first axis 16, the flexible member 39 is wrapped around the cam surface 38 of the cam member 35, and more particularly around the cam lobe 37 thereof, the spring 40 exerts on the board 13 a turning moment about the second axis 18 which tends to cuase turning of the board 13 about the second axis 18 in a clockwise direction as viewed in FIG. 4. However, under the influence of the spring 24 the force which is exerted by the bearing rollers 33 on the cam surface 34 of the further cam member 31 produces a turning moment on the further cam mem ber 31, and hence on the board 13, in a counterclockwise direction as viewed in FIG. 4, this turning moment on the board 13 in a counterclockwise direction about the second axis 18 substantially counterbalancing the above-described turning moment exerted on the board 13 by the spring 40 in the clockwise direction about the second axis 18.

Considering now the turning moments exerted about the first axis 16 relative to the support 11 with reference to FIG. 4, the weight of the arm 12 acts in a vertically downward direction substantially through the first axis 16 and the weight of the board 13 which acts, of course, in a vertically downward direction through the second axis 18 likewise acts in a vertically downward direction through the first axis 16. Thus, the combined weight of the board 13 and of the arm 12 substantially does not result in any tendency for the arm 12, and the board 13, pivotally to move in one direction or the other about the first axis 16. However, the spring 24 exerts on the farm 12 a turning moment about the first axis 16 which is in a clockwise direction, the arm 12 in the pivotal setting shown in FIG. 4 abutting against a stop member 73 which is fixed relative to the support 1 1 thereby to prevent pivotal movement of the arm 12, and the board 13, in a clockwise direction about the first axis 16 beyond the pivotal setting thereof shown in FIG. 4 under the influence of the clockwise turning moment hereinbefore described.

Referring now to FIG. 5 of the accompanying drawings in which the arm 12 is, about the first axis 16, in the same pivotal setting relative to the support 11 as is hereinbefore described with reference to FIG. 4, it will be noted that the board 13 has been pivotally moved relative to the arm 12 about the second axis 18 to the same pivotal setting in which the board 13 is shown in FIG. 3 i.e. the pivotal setting thereof in which the centre of gravity of the board 13 is disposed to the right of the vertical plane containing the second axis 18. Thus, with the board 13 in this pivotal setting shown in FIG. 5 there is a tendency for the board 13 under the influence of the weight thereof to pivot about the second axis 18 in a clockwise direction as viewed in FIG. 5. Furthermore, with the board 13 in this pivotal setting shown in FIG. 5 the flexible member 39 is wrapped, to a small extent which is scarcely noticeable in FIG. 5, around the cam surface 38 of the cam member 35, and more particularly the cam lobe 37 thereof, so that the spring 40 thus exerts on the board 13 a turning moment about the second axis 18 which is likewise in a clockwise direction. However, with the board 13 in this pivotal setting shown in FIG. 5 it will be noted that the spring 24 of the first counterbalancing system exerts, through the action of the roller bearings 33 on the cam surface 34 of the further cam member 31, a further turning moment on the the further cam member 31, and hence on the board 13, which is counterclockwise in a direction as viewed in FIG. 5, this counterclockwise turning moment serving substantially to counterbalance the above-described turning moments in a clockwise direction about the second axis 18.

Considering the turning moments about the first axis 16 relative to the support 11 with reference to FIG. 5, the weight of the arm 12 acts in a vertically downward direction through the first axis 16 so that the weight of the arm 12 substantially does not result in any tendency for the arm 12 pivotally to move in one direction or the other about the first axis 16, while the weight of the board 13 acts in a vertically down-ward direction disposed to the right of the vertical plane containing the first axis 16 so that the weight of the board 13 thus exerts a turning moment in a clockwise direction about the first axis 16. The spring 24 exerts on the arm 12 a turning moment about the first axis 16 which is also in a clockwise direction, the arm 12 again abutting against the stop member 73 thereby to prevent pivotal movement of the arm 12, and the board 13, in a clockwise direction about the first axis 16 beyond the pivotal setting thereof shown in FIG. 5 under the influence of the clockwise turning moments hereinbefore described.

Thus, within a predetermined range of angular settings of the board 13 about the second axis 18, namely, angular settings thereof between a first setting in which the board 13 is in a vertical plane and a second setting in which the board 13 is in a horizontal plane and is removed from said first setting, with the arm 12 in any angular setting thereof about the first axis 16 between an angular setting thereof in which the second axis 18 is disposed to one side of and in the same horizontal plane as the first axis 16 and an angular setting thereof in which the second axis 18 is disposed above and in the same vertical plane as the first axis 16, the algebraic sum of:

(W X d (F X d,,) F, X d is substantially zero, wherein W weight of the board 13,

F,, force exerted by the spring 40 of the second counterbalancing system on the counterbalancing cam member 35 thereof,

F force exerted by spring 24 of the first counterbalancing system on said further cam member 31 d perpendicular distance of the second axis 18 from the line of action of W through the centre of gravity of the board 13,

d perpendicular distance of the second axis 18 from the line of action F,,, and

d perpendicular distance of the second axis 18 from the line of action of F,,. Thus, within said predetermined range of angular settings the second counterbalancing system can be considered as substantially counterbalancing the turning moment exerted by the weight of the board 13 about the second axis 18 combined with the turning moment exerted by the first counterbalancing system aboutthe second axis 18 on the further cam member 31.

Likewise, although as hereinbefore described with reference to FIGS. 4 and 5 the stop member 73 is required in the conditions shown therein, within a predetermined range of angular settings of the arm 12 about the first axis 16, namely, angular settings thereof between a first setting in which the second axis 18 is disposed to one side of and in the same horizontal plane as the first axis 16 and a second setting in which the second axis 18 is disposed above and in the same vertical plane as the first axis 16, with the board 13 in any angular setting thereof about the second axis 18 between an angular setting'thereof in which the board is in a vertical plane and an angular setting thereof in which the board 13 is in a horizontal plane and is 90 removed from the first-mentioned setting thereof, the the algebraic sum of:

(W X d (W X d (P X 11;) is substantially zero, wherein W weight of the board 13,

W weight of the arm 12,

F c force exerted by the spring 24 of the first counterbalancing system on the arm 12,

d perpendicular distance of the first axis 16 from the line of action of W through the centre of gravity of the board 13,

d e perpendicular distance of the first axis 16 from the line of action of W through the centre of gravity of the arm 12, and

d perpendicular distance of the first axis 16 from the line of action of F Thus, within said predetermined range of angular settings the first counterbalancing system can be considered as substantially counterbalancing the turning moment exerted by the combined weight of the arm 12 and board 13 about the first axis 16, with variations in the turning moment about the first axis 16 resulting from variations in the angular setting of the third member being substantially compensated by variations in the first counterbalancing system as a result of the influence of the cam surface 34 of the further cam member 31 on the spring member of the first counterbalancing system. 7

In order, if desired, to permit the location of the centre of gravity of the board 13 to be varied an adjustment weight 74 may be movably mounted on the board 13, preferably on the main underface 19 thereof.

Referring again to FIG. 1 of the drawings, the drafting table according to the preferred embodiment of the invention as illustrated in the accompanying drawings further comprises a first braking mechanism for locking the pivotal connection of the arm 12 relative to the support 11 about the first axis 16, and a second braking mechanism for locking the pivotal connection of the board 13 relative to the arm 12 about the second axis 18.

As is most clearly shown in FIG. 6, the first braking mechanism comprises a friction disc 42 which is coaxially mounted on the tube 17 within the arm 12, and a bracket which is denoted generally by the reference numeral 43 and which is fixedly mounted within the arm 12, the bracket 43 being of generally U-shape with a slot 44 provided between the limbs of the bracket 43. The base 45 of the slot 44 is of inclined form and the disc 42 is disposed through the slot 44. A portion of one of the limbs of the bracket 43 which is in the form of a plug 46 of circular cross-section is screw-threadedly mounted within an aperture provided in said limb of the bracket 43 with an arm 47 being fixedly mounted on the outer end of the plug 46 for causing turning movement of the plug 46 relative to the remainder of the bracket 43, the screw-threaded connection between the plug 46 and the remainder of the bracket 43 being constituted by relatively coarse screw-threads so that on turning movement of the arm 47 through a relatively small angle the plug 46 is caused axially to move through a sufficient distance securely to clamp the disc 42 within the slot 44 as is hereinafter more fully explained. A portion of the other limb of the bracket 43, which is also in the form of a plug 48 of circular crosssection, is likewise screw-threadedly mounted within an aperture provided in said other limb of the bracket 43, the outerface of the plug 48 being provided with, for example, slotting 49 by means of which the plug 48 may, relative to the remainder of the bracket 43 be turned in the appropriate direction by a screw driver or other suitable tool (not shown) thereby axially to move the plug 48 and so adjust the effective width of the slot 44. The screw-threaded connection between the plug 48 and the remainder of the bracket 43 is preferably constituted by screw-threading which is substantially finer than the screw-threading between the plug 46 and the remainder of the bracket 43, and a set screw 50 is preferably mounted in said other limb of the bracket 43 for engagement with the plug 48 thereby to lock this plug 48 relative to the remainder of the bracket 43 in any desired setting thereof.

The second braking mechanism is identical to the first braking mechanism as hereinbefore described and like reference numerals are used to denote like parts with these reference numerals having prime marks where they are used to denote the parts of the second braking mechanism, except that whereas the disc 42 is mounted on the tube 17 the corresponding disc 42' is mounted on the tube 21 for turning movement therewith relative to the arm 12.

With particular reference to FIGS. 7 and 8 of the drawings, 51 denotes generally a housing which is secured to the main underface 19 of the board 13 adjacent to the front edge 52 thereof.

Mounted on this main underface 19 of the board 13 within the casing 51 is a substantially U-shaped bracket 53 the spaced limb portions 54 of which are provided with aligned apertures through which a pivot pin 55 is disposed. Pivotally mounted on this pivot pin 55 adjacent the opposite ends thereof are two actuating members denoted by the reference numerals 56 and 57. Each of the actuating members 56 and 57 comprises two spaced parallel'plates 58 which are together movable as a unit as the respective actuating member 56 or 57 is pivotally moved about the pivot pin 55, the ends of the plates 58 remote from the pivot pin 55 being interconnected by an anchoring device 59 to which one end of a flexible cable 60 is secured. The two cables 60 are disposed through an opening 61 provided in the casing 51, and the other ends of these cables 60 are secured to the ends of the arms 47 and 47', respectively, which are remote from the plugs 46 and 46.

A support block 62 is pivotally mounted on the pivot pin 55 between the actuating members 56 and 57, this support block 62 having mounted thereon a manually operable member which is denoted generally by the reference numeral 63 and which comprises a block 64 and a lever 65. The lever 65 is secured to the block 64 and projects from the casing 51 through an enlarged opening 66 provided therein, the lever 65 being disposed adjacent to the main underface 19 of the board 13 and adjacent to the front edge 52 thereof, thereby to facilitate actuation of the lever 65 by the user of the table 10. The mounting of the manually operable member 63 on the support block 62 is by means of a pivot pin 67 which permits the manually operable member 62 pivotally to be moved between selectively locatable positions comprising a position in which a side lug 68 presented by the block 64 underlies the actuating member 56 but a further side lug 69 also presented by the block 64 is disposed clear of the actuating member 57 (the position shown in FIG. 8), a position in which the side lug 68 underlies the actuating member 56 and the side lug 69 underlies the actuating member 57, and a position in which the side lug 68 is disposed clear of the actuating member 56 but the side lug 69 underlies the actuating member 57. A centrally located spring loaded ball assembly 70 is provided in the support block 62, the adjacent face of the block 64 being provided with three spaced detents 71 within the appropriate one of which the ball of the spring-loaded ball as sembly 70 is disposed when the manually operable member 63 is in each of the above-described selectively locatable positions, thereby releasably to restrain the manually operable member 63 in this position.

Manual movement of the lever 65 in the direction of the arrow D in FIG. 7 results in pivotal movement of the block 64, and of the support block 62, about the pivot pin 55 irrespective of which of the abovedescribed selectively locatable positions the manually operable member 63 occupies. Such pivotal movement of the block 64 results in one or both of the actuating members 56 and 57, depending of course on which of the above-described selectively locatable positions the manually operable member 63 occupies, being pivotally moved in the appropriate direction from a first position to a second position thereof.

Pivotal movement of the actuating members 56 as hereinbefore described results in turning movement, through the influence of the appropriate cable 60, of the arm 47 in the appropriate direction to cause axial movement of the plug 46 from the operative condition of the first braking mechanism in which the plug 46 is in secure clamping engagement with the disc 42 thereby to lock the pivotal connection of the arm 12 relative to the support 11 about the first axis 16 to an inoperative position thereof in which the plug 46 is no longer in clamping engagement with the disc 42 so that the pivotal connection of the arm 12 relative to the support 11 about the first axis 16 is unlocked. Likewise, pivotal movement of the actuating member 57 as hereinbefore described results in turning movement, through the influence of the appropriate cable 60, of the arm'47' in the appropriate direction to cause axial movement of the plug 46' from the operative condition of the second braking mechanism in which the plug 46' is in secure clamping engagement with the disc 42 thereby to lock the pivotal connection of the board 13 relative to the arm 12 about the second axis 18 to an inoperative condition thereof in which the plug 46' is no longer in clamping engagement with the disc 42' so that the pivotal connection of the board 13 relative to the arm 12 about the second axis 18 is unlocked.

When the lever 65 is subsequently released the arm 47, or the arm 47', or both of the arms 47 and 47, as the case may be, are automatically returned to their initial positions in which the first and second braking mechanisms are in the operative conditions thereof by means of springs 72 which act between the ends of the arms 47 and 47' and appropriate locations on the arm 12.

Thus, by means of the first and second braking mechanisms as hereinbefore described the pivotal connection of the arm 12 relative to the support 11 about the first axis 16, or the pivotal connection of the board 13 relative to the arm 12 about the second axis 18, or both of these pivotal connections may, by appropriate operation of the single lever 65 be disposed in a locked or unlocked condition.

While as hereinbefore described with reference to the accompanying drawings the lever 65 is appropriately actuated to operate the first braking mechanism, or the second braking mechanism, or both the first and second braking mechanisms, from the operative to the inoperative conditions thereof with these braking systems being automatically returned under the influence of the springs 72 to their initial operative conditions on release of the lever 65 it will, of course, be understood that in alternative embodiments (not shown) of the invention the appropriate actuation of the lever 65 may operate the first braking system, or the second braking system, or both the first and second braking systems,

from the inoperative to the operative conditions thereof, under the influence of the appropriate cable or cables 60, with the braking systems being automatically returned to their initial inoperative conditions under the influence of the springs 72. However, such alternative embodiments are not preferred since, in order to avoid the necessity for the user of the drafting table 10 manually to retain the lever 65 in the position in which the braking systems are in the operative conditions thereof while the drafting table 10 is in use, it would be necessary to incorporate a latching mechanism for retaining the lever 65 in this position.

While as hereinbefore described with reference to the accompanying drawings the cam member 22 of the first counterbalancing system is fixedly mounted relative to the support 11 it will be appreciated that by appropriate modification to the apparatus this cam member could instead be fixedly mounted relative to the arm 12 in which case, however, the end of the flexible member 23 which is fixedly secured at the location 27 would still be fixedly secured relative to the support 1 1. Likewise, while in the preferred embodiment of the invention as hereinbefore described with reference to the accompanying drawings the cam member 35 of the second counterbalancing system is fixedly secured relative to the drawing board 13 it will be appreciated that in alternative embodiments of the invention this cam member could, by appropriate modification of the apparatus, be fixedly secured relative to the arm 12, in which case the end of the flexible member 39 which is fixedly secured relative to the board 13 would still be fixedly secured relative to the board 13.

Furthermore, instead of the first and second braking mechanisms as hereinbefore described with reference to the accompanying drawings there may in alternative embodiments of the invention be provided other forms of braking mechanisms for locking the pivotal connection about the first axis 16 between the arm 12 and the support 11 and for locking the pivotal connection about the second axis 18 between the board 13 and the arm 12. Thus, for example, the braking mechanism may be constituted by band braking mechanisms, or drum braking mechanisms having either internal expandible brake shoes or external contractible brake shoes.

What I claim as my invention is:

1. Apparatus comprising a first member, a second member pivotally connected to the first member about a first horizontal axis, a third member pivotally connected to the second member about a second horizontal axis which is spaced from but parallel to the first axis, a first counterbalancing system between said first and second members, and a second counterbalancing system between said second and third members, each counterbalancing systeni comprising a counterbalancingcam member having an non-circular cam surface, and a spring biased member which is disposed around the cam surface of the counterbalancing cam member and which acts between the two members which are pivotally connected about the respective horizontal axis, with the counterbalancing cam member being fixed relative to one of said two members and with the spring biased member reacting against the other of said two members, wherein a further cam member having a non-circular cam surface is fixed relative to the third member with the spring biased member of the first counterbalancing system disposed around the cam surface of said further cam member, and wherein said counterbalancing cam of said first system and said further cam member are so configured that, within a predetermined range of angular settings of the third member about the second horizontal axis and of the second member about the first horizontal axis, the algebraic sum of:

(W X d +'(F X d,,) (F,, X d is substantially zero, and the algebraic sum of:

(W X d (W X 11,.) (F X d;) is substantially zero, in which W, weight of the third member,

W weight of the second member,

1" force exerted by the spring biased member of the second counterbalancing system on the counterbalancing cam member thereof,

F force exerted by the spring biased member of the first counterbalancing system on said further cam member,

F force exerted by the spring biased member of the first counterbalancing system on the second mem ber,

d perpendicular distance of the second horizontal axis from the line of action of W through the centre of gravity of the third member,

d perpendicular distance of the second horizontal axis from the line of action of F d perpendicular distance of the second horizontal axis from the line of action of P d perpendicular distance of the first horizontal axis from the line of action of W through the center of gravity of the third member,

d,, perpendicular distance of the first horizontal axis from the line of action of W through the centre of gravity of the second member, and

d perpendicular distance of the first horizontal axis from the line of action of F 2. Apparatus according to claim 1, wherein the apparatus comprises a drafting table, the first member comprising a fixed support, the'second member comprising an arm, and the third member comprising a drawing board.

3. Apparatus according to claim 1, wherein in the first counterbalancing system the counterbalancing cam member is fixed relative to the first member, one end of the spring biased member being fixed relative to the first member and the other end of the spring biased member being fixed relative to the second member, and wherein in the second counterbalancing system the cam member is fixed relative to the third member, one end of the spring biased member being fixed relative to the second member and the other end of the spring biased member being fixed relative to the third member.

4. Apparatus according to claim 1, wherein the spring biased member of the first counterbalancing sys' tem comprises a flexible member and a coil spring, one end of the flexible member being secured to the counterbalancing cam member of the first counterbalancing system and the other end of the flexible member being connected to said coil spring of the first counterbalancing system.

5. Apparatus according to claim 4, wherein the spring biased member of the first counterbalancing system further comprises a lever, one end of said lever being pivotally mounted to the second member about a horizontal axis which is spaced from but parallel to said first and second horizontal axes, and the other end of said lever being connected to the coil spring of the first counterbalancing system, with the lever between the ends thereof being in bearing contact with the cam surface of said further cam member.

6. Apparatus according to claim 16, wherein the spring biased member of the second counterbalancing system comprises a flexible member and a coil spring, one end of said flexible member being secured to the counterbalancing cam member of the second counterbalancing system and the other end of said flexible member being connected to said coil spring of the second counterbalancing system.

7. Apparatus according to claim 6, wherein the counterbalancing cam member of the second counterbalancing system is constituted by a cam member presenting two cam lobes, said one end of said flexible member of the spring biased member of the second counterbalancing system being secured to said cam member between the two cam lobes thereof.

8. Apparatus according to claim 5, wherein the further cam member is constituted by two spaced, paralell cam member portions each of which presents a portion of the cam surface of said further cam member, the lever of the spring biased member of the first counterbalancing system being partially movable between said two cam member portions, and the lever presenting between the ends thereof freely rotatable bearing rollers which are in bearing contact with said portions of the cam surface presented by said two cam member portions thereby to provide said bearing contact between the lever and the cam surface of said further cam.

9. Apparatus according to claim 1, wherein an adjustment weight is movably mounted on the third member for adjusting the location of the centre of gravity of the third member.

10. Apparatus according to claim 1, wherein a first braking mechanism is provided for locking the pivotal connection of the second member relative to the first member about the first horizontal axis, and a second braking mechanism is provided for locking the pivotal connection of the third member relative to the second member about the second horizontal axis, each of said braking mechanisms having an operative condition in which the pivotal connection between the respective members is locked and an inoperative condition in which the pivotal connection between the respective members is unlocked, an actuating member being provided for each of the braking mechanisms with each actuating member being connected to the respective braking mechanism and being movable between a first position in which the respective braking mechanism is in the operative condition thereof and a second position in which the respective braking mechanism is in the operative condition thereof, and a manually operable member being provided for movement of the actuating members between the first and second positions thereof with the manually operable member being selectively locatable for such movement of both actuating members, or of one of the actuating members, or of the other of the actuating members.

11. Apparatus according to claim 10, wherein the apparatus comprises a drafting table, the first member comprises a fixed support, the second member comprises an arm, and the third member comprises a drawing board, the drawing board having a mainunder face and a front edge, and the manually operable member being disposed adjacent to said main under face and front edge of the drawing board.

12. Apparatus according to claim 10, wherein the connections between the actuating members and the which has two spaced limbs defining therebetween a slot through which said disc is disposed, and a plug screw-threadedly mounted in one of said limbs and turnable for movement of the plug into and out of secure locking engagement with said disc, the braking mechanism, with said plug in secure locking engagement with said disc, being in the operative condition thereof, and, with said plug out of secure locking engagement with said disc, being in the inoperative condition thereof.

' 14. Apparatus according to claim 13, wherein a further plug is screw-threadedly mounted in the the other of said limbs of the bracket for adjustment of the effective width of said slot. 

1. Apparatus comprising a first member, a second member pivotally connected to the first member about a first horizontal axis, a third member pivotally connected to the second member about a second horizontal axis which is spaced from but parallel to the first axis, a first counterbalancing system between said first and second members, and a second counterbalancing system between said second and third members, each counterbalancing system comprising a counterbalancing cam member having an noncircular cam surface, and a spring biased member which is disposed around the cam surface of the counterbalancing cam member and which acts between the two members which are pivotally connected about the respective horizontal axis, with the counterbalancing cam member being fixed relative to one of said two members and with the spring biased member reacting against the other of said two members, wherein a further cam member having a non-circular cam surface is fixed relative to the third member with the spring biased member of the first counterbalancing system disposed around the cam surface of said further cam member, and wherein said counterbalancing cam of said first system and said further cam member are so configured that, within a predetermined range of angular settings of the third member about thE second horizontal axis and of the second member about the first horizontal axis, the algebraic sum of: (W3 X da) + (Fa X db) + (Fb X dc) is substantially zero, and the algebraic sum of: (W3 X dd) + (W2 X de) + (Fc X df) is substantially zero, in which W3 weight of the third member, W2 weight of the second member, Fa force exerted by the spring biased member of the second counterbalancing system on the counterbalancing cam member thereof, Fb force exerted by the spring biased member of the first counterbalancing system on said further cam member, Fc force exerted by the spring biased member of the first counterbalancing system on the second member, da perpendicular distance of the second horizontal axis from the line of action of W3 through the centre of gravity of the third member, db perpendicular distance of the second horizontal axis from the line of action of Fa, dc perpendicular distance of the second horizontal axis from the line of action of Fb, dd perpendicular distance of the first horizontal axis from the line of action of W3 through the center of gravity of the third member, de perpendicular distance of the first horizontal axis from the line of action of W2 through the centre of gravity of the second member, and df perpendicular distance of the first horizontal axis from the line of action of Fc.
 2. Apparatus according to claim 1, wherein the apparatus comprises a drafting table, the first member comprising a fixed support, the second member comprising an arm, and the third member comprising a drawing board.
 3. Apparatus according to claim 1, wherein in the first counterbalancing system the counterbalancing cam member is fixed relative to the first member, one end of the spring biased member being fixed relative to the first member and the other end of the spring biased member being fixed relative to the second member, and wherein in the second counterbalancing system the cam member is fixed relative to the third member, one end of the spring biased member being fixed relative to the second member and the other end of the spring biased member being fixed relative to the third member.
 4. Apparatus according to claim 1, wherein the spring biased member of the first counterbalancing system comprises a flexible member and a coil spring, one end of the flexible member being secured to the counterbalancing cam member of the first counterbalancing system and the other end of the flexible member being connected to said coil spring of the first counterbalancing system.
 5. Apparatus according to claim 4, wherein the spring biased member of the first counterbalancing system further comprises a lever, one end of said lever being pivotally mounted to the second member about a horizontal axis which is spaced from but parallel to said first and second horizontal axes, and the other end of said lever being connected to the coil spring of the first counterbalancing system, with the lever between the ends thereof being in bearing contact with the cam surface of said further cam member.
 6. Apparatus according to claim 16, wherein the spring biased member of the second counterbalancing system comprises a flexible member and a coil spring, one end of said flexible member being secured to the counterbalancing cam member of the second counterbalancing system and the other end of said flexible member being connected to said coil spring of the second counterbalancing system.
 7. Apparatus according to claim 6, wherein the counterbalancing cam member of the second counterbalancing system is constituted by a cam member presentiNg two cam lobes, said one end of said flexible member of the spring biased member of the second counterbalancing system being secured to said cam member between the two cam lobes thereof.
 8. Apparatus according to claim 5, wherein the further cam member is constituted by two spaced, paralell cam member portions each of which presents a portion of the cam surface of said further cam member, the lever of the spring biased member of the first counterbalancing system being partially movable between said two cam member portions, and the lever presenting between the ends thereof freely rotatable bearing rollers which are in bearing contact with said portions of the cam surface presented by said two cam member portions thereby to provide said bearing contact between the lever and the cam surface of said further cam.
 9. Apparatus according to claim 1, wherein an adjustment weight is movably mounted on the third member for adjusting the location of the centre of gravity of the third member.
 10. Apparatus according to claim 1, wherein a first braking mechanism is provided for locking the pivotal connection of the second member relative to the first member about the first horizontal axis, and a second braking mechanism is provided for locking the pivotal connection of the third member relative to the second member about the second horizontal axis, each of said braking mechanisms having an operative condition in which the pivotal connection between the respective members is locked and an inoperative condition in which the pivotal connection between the respective members is unlocked, an actuating member being provided for each of the braking mechanisms with each actuating member being connected to the respective braking mechanism and being movable between a first position in which the respective braking mechanism is in the operative condition thereof and a second position in which the respective braking mechanism is in the operative condition thereof, and a manually operable member being provided for movement of the actuating members between the first and second positions thereof with the manually operable member being selectively locatable for such movement of both actuating members, or of one of the actuating members, or of the other of the actuating members.
 11. Apparatus according to claim 10, wherein the apparatus comprises a drafting table, the first member comprises a fixed support, the second member comprises an arm, and the third member comprises a drawing board, the drawing board having a main under face and a front edge, and the manually operable member being disposed adjacent to said main under face and front edge of the drawing board.
 12. Apparatus according to claim 10, wherein the connections between the actuating members and the respective braking mechanisms are constituted by flexible cables by means of which the respective braking mechanisms may be urged towards one of the conditions consisting of the operative and inoperative conditions thereof, springs biasing the respective braking mechanisms towards the other of the conditions consisting of the respective and inoperative conditions thereof.
 13. Apparatus according to claim 10, wherein each braking mechanism comprises a disc which is mounted on one of the respective members, a bracket which is mounted on the other of the respective members and which has two spaced limbs defining therebetween a slot through which said disc is disposed, and a plug screw-threadedly mounted in one of said limbs and turnable for movement of the plug into and out of secure locking engagement with said disc, the braking mechanism, with said plug in secure locking engagement with said disc, being in the operative condition thereof, and, with said plug out of secure locking engagement with said disc, being in the inoperative condition thereof.
 14. Apparatus according to claim 13, wherein a further plug is screw-threadedly mounted in the the other of said limbs of the bracket for adjustment of the effective width of said slot. 